Carrier aggregation is a feature of Evolved Universal Mobile Telecommunications System (UMTS) Universal Terrestrial Radio Access (E-UTRA) (Long Term Evolution (LTE)) by which it is possible to increase user throughput and cell capacity by aggregating carriers (radio channels) to increase the total bandwidth. Combining E-UTRA carriers of bandwidths between 1.4 and 20 Megahertz (MHz) within an operating band is referred to as intra-band carrier aggregation, whereas combining E-UTRA carriers in different operating bands is referred to as inter-band carrier aggregation. Combinations of intra- and inter-band aggregation are also possible. Currently, up to five carriers can be combined to generate an aggregate bandwidth of up to 100 MHz.
One component carrier in a combination of bands (i.e., a band combination) used for carrier aggregation with respect to a User Equipment (UE) acts a Primary Component Carrier (PCC) by which the UE performs initial connection establishment and handovers. One or more additional component carriers on other frequency carriers are designated as Secondary Component Carriers (SCCs) for the UE. Carrier aggregation can also be used in the downlink only. In this case, uplink transmissions from the UE occur over the PCC in one of the operating bands, while downlink transmissions to the UE occur simultaneously over the PCC as well as the SCC in different operating bands. In this scenario, there is no uplink transmission on the SCC.
There are currently 28 Frequency Division Duplex (FDD) and 8 Time Division Duplex (TDD) bands available for E-UTRA (LTE) networks. A combination of at least two carriers within an operating band or in different operating bands constitutes a band combination, and each carrier in combination is denoted as a component carrier. Thus, for inter-band carrier aggregation, there are many possible band combinations. Intra/inter-band carrier aggregation of at least two component carriers is also referred to as a band combination in the 3rd Generation Partnership Program (3GPP) specifications, but in this disclosure it is sufficient to discuss inter-band carrier aggregation.
Each band combination of two or more component carriers uses a dedicated solution in the UE, and each UE can support a limited set of band combinations. The band combinations supported by the UE are indicated in UE capability information sent to a Base Station (BS) during a UE capability exchange between the BS and a corresponding Mobility Management Entity (MME) or between the BS and the UE. The UE capability exchange occurs when a connection between the BS and the UE is configured/established. Specifically, in this capability exchange, an indication is provided to the BS of which of the 28 FDD and 8 TDD bands are supported by the UE. The band combinations supported by the UE are then the combinations of the operating bands supported by the UE. The UE can support a finite set of operating bands and band combinations.
One problem that arises is that the ability to provide carrier aggregation to, e.g., increase throughput for a UE is limited by the finite set of band combinations that are supported by the UE. For instance, when the UE is roaming, the band combinations supported by the UE may not be supported by the network. As a result, even though the UE supports carrier aggregation, the network may be unable to provide carrier aggregation for the UE, since the network may not support the band combination(s) that are supported by the UE. Thus, there is a need for systems and methods for enabling carrier aggregation when a band combination of a UE is not supported by the network.